The present technology relates to non-volatile memory systems.
Semiconductor memory devices have become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices.
A charge-storing material such as a conductive floating gate or a non-conductive charge-trapping material can be used in some types of memory cells to store a charge which represents a data state. The memory cell may be a transistor in which the charge-storing material may reside between a channel and a control gate. The charge-storing material may be insulated from both the channel and the control gate. The charge-storing material can be arranged vertically in a three-dimensional (3D) stacked memory structure, or horizontally in a two-dimensional (2D) memory structure. One example of a 3D memory structure is the Bit Cost Scalable (BiCS) architecture which comprises a stack of alternating conductive and dielectric layers.
Some types of memory cells may be programmed by altering the charge in the charge-storing material, thereby altering the threshold voltage (Vth) of the memory cell transistor. In one technique, a program voltage is applied to the control gate with a low voltage in the channel to add charge to the charge-storing material. After applying the program voltage, the memory cell's threshold voltage is tested by applying a verify voltage to the control gate, and testing whether the memory cell conducts a significant current. Additional program voltages may be applied to control gate, followed by verify voltages, until the memory cell's threshold current is within a target range.
Other types of memory cells can be programmed to different resistance levels. For example, resistive random access memory (ReRAM) can be reversibly switched between a high resistance state and a low resistance data state. In some memory cells, a ReRAM cell can be reversibly switched between more than two different resistance states. Each resistance state can correspond to a range of resistances.
A memory cell may be read by applying one or more read reference signals to it to determine what range its threshold voltage, resistance, or other physical property falls within. Often, memory cells are programmed in a basic unit referred to as a “page.” The page contains user data and overhead data, in some techniques. Overhead data typically includes parity bits of an Error Correction Code (ECC) that have been calculated from the user data. An error correction controller (ECC) engine encodes user data by adding the parity bits to form a codeword. The ECC engine decodes and error corrects the codeword read from the non-volatile memory system.